This invention relates to the safe handling of tetrafluoroethylene under confinement.
Tetrafluoroethylene (TFE) is a gas at ambient temperature (20.degree. C.), but can be liquified by sufficient pressurization in a container, to provide a relatively large quantity of the tetrafluoroethylene within the container. For economic or space-saving reasons, it is desireable to store or transport TFE as a liquid because a greater amount of TFE can be stored in a given space.
TFE is flammable when mixed with air and exposed to such conditions as atmospheric pressure and contact with a hot surface at a temperature of about 240.degree. C. The greater danger presented by TFE, however, is the explosivity of TFE gas vaporizing from TFE liquid contained under pressure. This explosivity results from the decomposition of TFE to CF.sub.4 and C. This explosive decomposition can result from exposure of the TFE vapor to high temperatures. Also, the susceptability to explosion increases with increasing TFE pressure. Oxygen can cause autopolymerization of TFE, which can in turn cause sufficient localized heating within the container to cause the TFE vapor above the liquid within the container to explode. The explosivity of TFE may not depend on the presence of air, however, since air will normally be excluded (purged) from the container of liquid TFE. Even in the absence of air, exposure of the TFE vapor within the container to a spark, such as caused by discharge of static electricity, a hot metal surface, such caused by metal surfaces rubbing together, or an external fire can cause the TFE vapor to explode. Thus, for example, a saturated TFE vapor can explode at temperatures of -16.degree. C. or greater when under a pressure of at least 150 psia (1032 KPa), while an unsaturated TFE vapor can explode at 25.degree. C. and 115 psia (790 KPa).
Heretofore, the problem of TFE explosivity during transportation has been solved by adding HCl to the liquid tetrafluoroethylene within the pressurized container, the proportion of TFE to HCl being about 33 mol % TFE and 67 mol % of HCl. This mixture forms an azeotrope, so as tetrafluoroethylene vaporizes, so does HCl in about the same proportion. The presence of HCl with the TFE in the vapor state renders the TFE non-ignitable, and therefore non-explosive, in the absence of air. While this solution solves the problem of the explosion hazard of the tetrafluoroethylene, this solution has both the problem of disposal of the large amount of HCl present with the tetrafluoroethylene, when the tetrafluoroethylene is to be used, e.g. in polymerization to polytetrafluoroethylene and other polymers, as well as the HCl toxicity problem in the event of an accidental release. The disposal problem is also true for the use of trifluorotrichloroethane or a perfluoroalkane, as disclosed in Japanese Kokai 45-39082 (Dec. 9, 1970) and 57-48096 (Oct. 14, 1982), respectively, in admixture with liquid tetrafluoroethylene for safe storage, unless they are specifically desired as components or carrier solvents for the specific use.
U.S. Pat. No. 4,365,102 discloses the heating of gaseous mixtures of TFE and CO.sub.2 at temperatures of 950.degree. to 1500.degree. C. in order to make CF.sub.4 and C.sub.2 F.sub.6, with the CO.sub.2 being disclosed to act as a diluent for the TFE to reduce the chance of explosion due to the decomposition of TFE (at CO.sub.2 molecular concentrations greater than 20 mol % CO.sub.2) at the reaction temperature and to act endothermically to moderate the temperature rise coming from the heat of the chemical reaction. U.S. Pat. No. 3,873,630 discloses pyrolysis of a TFE/CO.sub.2 (8 to 70 mol % CO.sub.2) gaseous mixture at 700.degree. to 900.degree. C. and essentially atmospheric pressure to obtain hexafluoropropylene. It also discloses that &gt;8 mol % CO.sub.2 is needed in the gaseous feed to the pyrolysis furnace to render TFE non-explosive at essentially one atmosphere pressure, and that &gt;60 mol % CO.sub.2 is needed in the feed to render the TFE/CO.sub.2 feed mixture non-flammable in case of leakage to the atmosphere.
Asahi Garasu Kenkyu Hokoku 38 no.1:115-22 (1988) entitled "Repressing Effects of Diluent Gases on the Disproportionation Reaction of Tetrafluoroethylene" discloses bringing together TFE with various diluent gases, including inert gases, notably N.sub.2 and CO.sub.2, for a short period of time at initial pressures of 157 to 228 psia (1080-1569 KPa) and undisclosed temperature and then igniting the resultant mixture, with the result being explosion of the mixtures at lower concentrations of diluent gas (15 mol % CO.sub.2) but not explosion at higher concentration (18 mol % CO.sub.2).